Nitriles, such as acrylonitrile and methacrylonitrile, have been industrially produced as important intermediates for the preparation of fibers, synthetic resins, synthetic rubbers, and the like. The most popular method for producing such nitrites is to subject an olefin such as propene or isobutene to a catalytic reaction with ammonia and oxygen in the presence of a catalyst in a gaseous phase at a high temperature. Known catalysts for conducting this reaction include a Mo—Bi—P—O catalyst, a V—Sb—O catalyst, an Sb—U—V—Ni—O catalyst, a Sb—Sn—O catalyst, a V—Sb—W—P—O catalyst and a catalyst obtained by mechanically mixing a V—Sb—W—O oxide and a Bi—Ce—Mo—W—O oxide. However, in view of the price difference between propane and propene or between isobutane and isobutene, attention has been drawn to the development of a method for producing acrylonitrile or methacrylonitrile by an ammoxidation reaction wherein a lower alkane, such as propane or isobutane, is used as a starting material, and it is catalytically reacted with ammonia and oxygen in a gaseous phase in the presence of a catalyst.
In particular, U.S. Pat. No. 5,281,745 discloses a method for producing an unsaturated nitrile comprising subjecting an alkane and ammonia in the gaseous state to catalytic oxidation in the presence of a catalyst which satisfies the conditions:
(1) the mixed metal oxide catalyst is represented by the empirical formulaMoaVbTecXxOnwherein X is at least one element selected from the group consisting of niobium, tantalum, tungsten, titanium, aluminum, zirconium, chromium, manganese, iron, ruthenium, cobalt, rhodium, nickel, palladium, platinum, antimony, bismuth, boron and cerium and, when a=1, b=0.01 to 1.0, c=0.01 to 1.0, x=0.01 to 1.0 and n is a number such that the total valency of the metal elements is satisfied; and
(2) the catalyst has X-ray diffraction peaks at the following angles (±0.3°) of 2θ in its X-ray diffraction pattern: 22.1°, 28.2°, 36.2°, 45.2°and 50.0°.
Similarly, Japanese Laid-Open Patent Application Publication No. 6-228073 discloses a method of nitrile preparation comprising reacting an alkane in a gas phase contact reaction with ammonia in the presence of a mixed metal oxide catalyst of the formulaWaVbTecXxOnwherein X represents one or more elements selected from niobium, tantalum, titanium, aluminum, zirconium, chromium, manganese, iron, ruthenium, cobalt, rhodium, nickel, palladium, platinum, antimony, bismuth, indium and cerium and, when a=1, b=0.01 to 1.0, c=0.01 to 1.0, x=0.01 to 1.0 and n is determined by the oxide form of the elements.
Unsaturated carboxylic acids such as acrylic acid and methacrylic acid are industrially important as starting materials for various synthetic resins, coating materials and plasticizers. Commercially, the current process for acrylic acid manufacture involves a two-step catalytic oxidation reaction starting with a propene feed. In the first stage, propene is converted to acrolein over a modified bismuth molybdate catalyst. In the second stage, acrolein product from the first stage is converted to acrylic acid using a catalyst composed of mainly molybdenum and vanadium oxides. In most cases, the catalyst formulations are proprietary to the catalyst supplier, but, the technology is well established. Moreover, there is an incentive to develop a single step process to prepare the unsaturated acid from its corresponding alkene. Therefore, the prior art describes cases where complex metal oxide catalysts are utilized for the preparation of unsaturated acid from a corresponding alkene in a single step.
Japanese Laid-Open Patent Application Publication No. 07-053448 discloses the manufacture of acrylic acid by the gas-phase catalytic oxidation of propene in the presence of mixed metal oxides containing Mo, V, Te, O and X wherein X is at least one of Nb, Ta, W, Ti, Al, Zr, Cr, Mn, Fe, Ru, Co, Rh, Ni, Pd, Pt, Sb, Bi, B, In, Li, Na, K, Rb, Cs and Ce.
Commercial incentives also exist for producing acrylic acid using a lower cost propane feed. Therefore, the prior art describes cases wherein a mixed metal oxide catalyst is used to convert propane to acrylic acid in one step.
U.S. Pat. No. 5,380,933 discloses a method for producing an unsaturated carboxylic acid comprising subjecting an alkane to a vapor phase catalytic oxidation reaction in the presence of a catalyst containing a mixed metal oxide comprising, as essential components, Mo, V, Te, O and X, wherein X is at least one element selected from the group consisting of niobium, tantalum, tungsten, titanium, aluminum, zirconium, chromium, manganese, iron, ruthenium, cobalt, rhodium, nickel, palladium, platinum, antimony, bismuth, boron, indium and cerium; and wherein the proportions of the respective essential components, based on the total amount of the essential components, exclusive of oxygen, satisfy the following relationships:    0.25<r(Mo)<0.98, 0.003<r(V)<0.5, 0.003<r(Te)<0.5 and 0.003<r(X)<0.5, wherein r(Mo), r(V), r(Te) and r(X) are the molar fractions of Mo, V, Te and X, respectively, based on the total amount of the essential components exclusive of oxygen.
The mixed metal oxide catalysts useful in the preparation of unsaturated carboxylic acids and unsaturated nitriles, as delineated above, can form more than one phase from the same starting materials under the same conditions. Often one phase performs better than the others, so it is desirable to prepare a catalyst that contains that phase exclusively, with the other phases substantially absent.
The aforementioned mixed metal oxide catalysts useful in the preparation of unsaturated carboxylic acids and unsaturated nitriles form at least three phases: a hexagonal phase (phase A), which is active but relatively unselective; an orthorhombic phase (phase B), which is active and selective; and a third phase (phase C) which is still poorly charcterized. It is desirable to form the orthorhombic phase (phase B) selectively.
Two methods of forming the orthorhombic phase (phase B) with substantially reduced content of the hexagonal phase (phase A) are known. The first method involves the extraction of a mixed phase catalyst with a suitable solvent. In particular, Japanese Laid-Open Patent Application Publication No. 10-330343 discloses the washing of a mixed metal oxide of the formulaMoaVbSbcXxOn                wherein X is at least one metal element selected from Ti, Zr, Nb, Ta, Cr, W, Mn, Fe, Ru, Co, Rh, Ir, Ni, Pd, Pt, Cu, Ag, Zn, In, Sn, Pb, Bi, Ce and alkaline earth metals,        wherein, when a=1, 0.02≦b≦0.99, 0.001≦c≦0.9, 0≦x≦0.89, 0.1≦c/b≦0.80 and n is a value determined by the oxidation state of the other elements,with a solvent selected from aqueous oxalic acid, ethylene glycol or aqueous hydrogen peroxide. The so-formed catalyst is used for the ammoxidation of alkanes to form nitriles. Japanese Laid-Open Patent Application Publication No. 11-043314 discloses the washing of a mixed metal oxide of the formulaMoaVbSbcXxOn        wherein X is at least one metal element selected from Ti, Zr, Nb, Ta, Cr, W, Mn, Fe, Ru, Co, Rh, Ir, Ni, Pd, Pt, Cu, Ag, Zn, In, Sn, Pb, Bi, Ce and alkaline earth metals,        wherein, when a=1, 0.02≦b≦0.99, 0.001≦c≦0.9,0≦x≦0.89, 0.1≦c/b≦0.80 and n is a value determined by the oxidation state of the other elements,with at least one solvent selected from an aqueous solution of an organic acid, an alcohol, an aqueous solution of an inorganic acid or an aqueous solution of hydrogen peroxide. The so-formed material is indicated to be useful in such applications as electronic materials, electrode materials, mechanical inorganic materials and as catalysts in petrochemistry, etc. In particular, use as a catalyst in the oxidative dehydrogenation of ethane to produce ethylene is exemplified. While this methodology allows isolation of the orthorhombic phase, it is undesirable because about one-third of the original sample is lost in the extraction. The second method involves the hydrothermal synthesis of the catalyst precursor (Watanabe, et al., “New Synthesis Route for Mo—V—Nb—Te Mixed Oxides Catalyst for Propane Ammoxidation”, Applied Catalysis A: General, Vol. 194-195, pgs. 479-485 (2000)). This gives, after calcination, a product enriched in the orthorhombic phase (phase B) but still containing the hexagonal phase (phase A).        
It has now been found that the orthorhombic phase (phase B) can be prepared selectively, in quantitative yield, by seeding the catalyst precursor solution with orthorhombic phase (phase B) material.